I. Field of the Invention
The present invention relates generally to intravascular devices for treating certain medical conditions, and, more particularly, relates to an intravascular flow restrictor for reducing blood pressure down stream of the location where the flow restrictor is placed.
II. Description of the Prior Art
In the normal heart, the right side pumps blood to the lungs, which is a relatively easy task, while the left side of the heart has the more difficult job of pumping blood all around the body. As a result, the pressure in the left ventricle (pumping chamber) is generally about five times that in the right ventricle, and the wall of the left ventricle is thicker than that of the right. There are a number of heart defects in which there is excessive blood flow to the lungs. Many defects that involve holes in the septum allow blood to flow from the high pressure left side of the heart to the lower pressure right side. This results in an increase in the pressure on the right and causes too much blood to be pumped to the lungs. The body's natural reaction to this is to constrict or narrow the blood vessels in the lungs in an effort to limit this excess blood flow. Over a period of time, this narrowing of the pulmonary arteries causes a thickening of the pulmonary arteries due to the increased workload, which leads ultimately to closure of smaller lung arteries which further reduces the blood flow into the lungs.
There is less and less left to right shunting of blood into the pulmonary arteries, and eventually the resistance is such that the shunt is reversed, i.e., right to left shunting occurs. This process is called pulmonary vascular disease and ultimately results in low oxygen levels and cyanosis and increased hemoglobin levels in the blood of the patient. It is the damage caused by prolonged pulmonary hypertension that generally prohibits late repair of cardiac defects. As children with Down's syndrome have a propensity to develop pulmonary vascular disease due to the fact that they tend to have larger holes in the heart, fewer small lung arteries and smaller airways, surgical repair is generally carried out fairly early in life, although timing will vary depending on the exact heart defect.
For example, in the case of ventricular septal defects (VSD), especially where there are multiple openings, it may not be possible to surgically close the defects. In the case of neonates, they may not be strong enough to survive an open-heart procedure required to repair multiple “Swiss cheese” septal defects. If an infant with VSD develops symptoms of congestive heart failure in the first few months of life, less traumatic palliative surgery may be attempted. Palliative surgery reduces the damage of the defect without correcting the underlying cause. One such palliative treatment is pulmonary artery (PA) banding. In the case of VSD, PA banding increases the resistance to blood flow through the pulmonary artery, preventing excessive shunting of blood from the left ventricle through the defects to the right ventricle.
In the case of an infant or young child with abnormally elevated pressure in the pulmonary artery, surgery is often considered too dangerous, but pulmonary banding may be effective. This procedure requires the surgeon to place a restrictive band around the pulmonary artery, thus reducing the blood flow into the lungs, and preventing the need for the body to form its own restriction. If successful, the normal development of pulmonary hypertension may be slowed or stopped, and surgical repair of the hole may be possible at a later date.
PA banding surgery, while less traumatic than open- heart surgery, still requires a thoracotomy to expose the pulmonary artery so that a constrictive band can be sutured around the pulmonary artery. The PA band reduces the diameter of the pulmonary artery and thereby restricts the amount of blood pumped into the lungs. Such an operation may reduce the blood flow from one-half to one-third of its previous volume. Pulmonary artery blood pressure distal to the band is reduced as a result of the volume restriction usually to about 50%–70% of the pulmonary artery pressure prior to banding.
While pulmonary artery banding is less risky than open heart surgery, it still carries the usual risks of surgery, such as bleeding, infection, pulmonary embolism, heart failure, etc. The special risk of the pulmonary artery banding procedure is making the band too tight or too loose. If it is too tight, too little blood will flow to the lungs and patient may become blue. If it is too loose, it will not eliminate the congestion of the lungs and will not protect the lungs from injury and pulmonary vascular disease.
Thus, a need exists for a non-surgical procedure, which is less traumatic than current procedures involving pulmonary artery banding, for restricting blood flow to the lungs in patients having congenital cardiac conditions which may cause pulmonary vascular disease such as, for example, left sided hypo plastic syndrome where flow restrictions are placed into the individual pulmonary arteries. The present invention meets that need without the risk of surgery, producing pain or large scar of the chest.